High capacity digital microwave beam transmission requiring a large bandwidth per channel is particularly subject to selective fading due to multi-path propagation. The passband required for transmitting each channel can, however, be reduced by using high spectrum efficiency multi-state modulation, thereby optimizing utilization of predetermined frequency allocations. Unfortunately, the complexity of such forms of modulation makes them sensitive to distortion. Transmission degradation gives rise to the appearance of errors, thereby degrading the quality, and in severe cases the availability, of a link.
Various remedies may be proposed:
frequency diversity: i.e. switching from a disturbed channel to a redundant channel used as a spare. Unfortunately, frequency diversity is insufficient when several channels are disturbed simultaneously; PA1 space diversity, which may be constituted by diversity at maximum power, by diversity at minimum distortion, or by diversity by baseband switching. Space diversity is effective (although limited to correcting trouble due to propagation), but it is costly in antennas and consequently in locations on microwave towers; PA1 self-adaptive equalization in baseband, i.e. full processing of the signal (in amplitude and in phase), capable of providing correction beyond that required for selective fading due to multiple paths. Such equalization is described in an article entitled "Self-adaptive baseband equalizers for digital microwave beams" by O. de Luca in the Thomson-CSF Technical Journal, volume 16, No. 1, March 1984; PA1 self-adaptive equalization at intermediate frequency. Unlike equalization in baseband, both of the paths in quadrature are processed simultaneously, thereby making it possible to obtain structures which are less complicated and thus cheaper in analog embodiments. An equalizer of this type based on the principle of the transversal filter, makes it possible to correct the distortions to which the signal is subject, and in particular those due to multiple paths.
The object of the invention is to optimize the structure of a transversal filter operating at intermediate frequency and used in a time equalizer.